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Fossils & Evolution Ch. 21 Ch. 2—Key concepts Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding.

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Presentation on theme: "Fossils & Evolution Ch. 21 Ch. 2—Key concepts Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding."— Presentation transcript:

1 Fossils & Evolution Ch. 21 Ch. 2—Key concepts Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding of intraspecific variation is necessary for correct identification Ontogenetic variation occurs during an individual’s lifespan Population variation occurs among individuals within a given population

2 Fossils & Evolution Ch. 22 Ch. 2—Key terms Ontogeny; ontogenetic variation Population variation Types of skeletal growth –Addition; accretion; molting; modification; combination Isometric vs. allometric growth Principle of similitude Ecophenotypic variation Sexual dimorphism

3 Fossils & Evolution Ch. 23 Ontogenetic variation Ontogeny = the life history of an individual (both embryonic and post-natal) Understanding ontogeny is important because growth stages of an individual may be so different that they are hardly recognizable as the same species

4 Fossils & Evolution Ch. 24 Types of skeletal growth 1.Accretion (enlargement) of existing parts 2.Addition of new parts 3.Molting 4.Modification 5.Combinations (mixed growth strategies)

5 Fossils & Evolution Ch. 25 Skeletal growth—Accretion Accretion = adding new material to an existing shell Allows uninterrupted use of shell and more or less continuous growth Disadvantage is that adult shape is somewhat constrained by juvenile shape Example: bivalve growth

6 Fossils & Evolution Ch. 26 Bivalve accretion

7 Fossils & Evolution Ch. 27 Skeletal growth—Addition of new parts Echinoderms may grow simply by adding new plates to their calyx or new columnals to their stalk Example: crinoid stalk –Large columnals added just beneath calyx –Smaller columnals added between larger ones –Alternation of different sizes allows increased flexibility

8 Fossils & Evolution Ch. 28 Crinoid stalk (addition)

9 Fossils & Evolution Ch. 29 Skeletal growth—Molting Molting = periodic shedding of an exoskeleton followed by growth of a new, larger one Advantage: Shape of adult organism not constrained by shape of juvenile stages Disadvantages are (1) vulnerable period during the molt itself; (2) significant metabolic cost of repeatedly replacing entire skeleton Example: trilobites

10 Fossils & Evolution Ch. 210 Trilobite molting Instars = growth stages between molts

11 Fossils & Evolution Ch. 211 Molting (cont.) Molting produces growth in a series of discrete episodes (not continuous)—Instars from different growth stages form distinct morphologic clusters instars

12 Fossils & Evolution Ch. 212 Skeletal growth—Modification Modification = process of replacement and re-formation of skeletal material, allowing size increase as well as changes in shape and structure Skeletal form of adult is not strongly constrained by skeletal form of juvenile No vulnerable stage (as in molting) Example: vertebrate bones

13 Fossils & Evolution Ch. 213 Skeletal growth—Mixed strategies Some organisms employ combinations of growth strategies Example: coiled cephalopod grows by accretion along leading edge of shell and also by periodic addition of septa

14 Fossils & Evolution Ch. 214 Combined growth strategy (coiled cephalopod) periodic addition of new septa continuous accretion of new material along leading edge of shell

15 Fossils & Evolution Ch. 215 Recognizing and describing ontogenetic change Biologists can directly observe ontogenetic change, but paleontologists cannot Two main approaches to studying ontogenetic changes in fossil material: –Growth series of specimens representing different developmental stages (as in successive trilobite instars) –Adult specimens whose development is recorded by growth lines or newly added parts (as in bivalve example)

16 Fossils & Evolution Ch. 216 Recognizing and describing ontogenetic change Approach depends on the kinds of fossils being studied: –Cannot use adult specimens to study ontogeny in animals that grow through molting or modification

17 Fossils & Evolution Ch. 217 Example 1: Brachiopod ontogeny Length and width measurements performed on large (~75) population of specimens of all sizes Plot of length vs. width suggests change in shape during growth –Small individuals are wider than long –Large individual are longer than wide

18 Fossils & Evolution Ch. 218 Brachiopod example: Length vs. width Growth Series: scatter of data points suggests change in shape during growth

19 Fossils & Evolution Ch. 219 Example: Brachiopod ontogeny A more definitive understanding of brachiopod ontogeny can be achieved by plotting growth curves for individual specimens (by measuring along growth lines)

20 Fossils & Evolution Ch. 220 Brachiopod example: Length vs. width Individual ontogeny: growth curves for single specimens confirm change in shape, AND allow estimate of variation among individuals

21 Fossils & Evolution Ch. 221 Types of growth Isometric = no change in shape during ontogeny (ratio between parts does not change as size increases) –Relatively uncommon Anisometric (allometric) = change in shape during ontogeny (ratio between parts changes as size increases) –Relatively common

22 Fossils & Evolution Ch. 222 Types of growth (cont.) Consider two body parts, X and Y As organism grows, relationship between X and Y is given as: In isometric growth, a = 1 (linear equation) In anisometric growth, a = 1 (curve) Y = bX a

23 Fossils & Evolution Ch. 223 Isometric growth

24 Fossils & Evolution Ch. 224 Anisometric growth

25 Fossils & Evolution Ch. 225 Why is anisometric growth common? Anisometric growth is necessary in most organisms because volume (body mass) increases as the cube of linear size increase Example: bone strength is proportional to cross- sectional area of bone –As linear dimensions of bone doubles, cross-sectional area is squared, but body mass is cubed –Body weight increases faster than relative strength of supporting bones This scaling inequality is “principle of similitude”

26 Fossils & Evolution Ch. 226 “Principle of similitude” 2 10 2 Cross-sectional area = 4 Volume = 40 4 4 20 Cross-sectional area = 16 Volume = 320

27 Fossils & Evolution Ch. 227 Anisometry of pelycosaur femurs (note different shapes as well as different sizes) decreasing size of animal

28 Fossils & Evolution Ch. 228 Population variation Variation among individuals within a population is called population variation Sources of population variation are: –Genetic differences among individuals –Ecophenotypic variation –Sexual dimorphism –Taphonomic effects

29 Fossils & Evolution Ch. 229 Populations Biologic definition of population = “a group of individuals of the same species living close enough together that each individual of a given sex has a chance of mating with an individual of the other sex” –“breeding population” Populations are characterized by a single gene pool –Gene flow occurs when two or more populations interbreed

30 Fossils & Evolution Ch. 230 Genetic variation: Alternation of generations in forams “megalospheric” (asexually produced) “microspheric” (sexually produced)

31 Fossils & Evolution Ch. 231 Ecophenotypic variation Variation among individuals as a consequence of differences in their environments: –Nutrition –Exposure to sunlight (plants; animals with phtotsynthesizing symbionts) –Space (crowding) –Environmental stability

32 Fossils & Evolution Ch. 232 Sexual dimorphism in ammonoids dimorphic pair dimorphic pair

33 Fossils & Evolution Ch. 233 Fossil populations Not as easy to work with as biologic (living) populations Sources of difficulty –Sedimentary mixing (reworking; bioturbation) Time-averaging; loss of temporal resolution –Preservation bias Distortion Dissolution (reduces observable variation) Post-mortem sorting

34 Fossils & Evolution Ch. 234 Structural distortion of bivalve shapes undeformed shape direction of rock cleavage

35 Fossils & Evolution Ch. 235 Effects of selective post-mortem transport

36 Fossils & Evolution Ch. 236 Fossil populations (cont.) Additional example of population “biasing” by selective transport Devonian brachiopods –Leptocoelia (879 pedicle; 893 brachial) –Platyorthis (561 pedicle; 548 brachial) –Leptostrophia (378 pedicle; 35 brachial) untransported, or not selectively transported


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